Heating, ventilation, and/or air conditioning (HVAC) systems are often used to control the comfort level within a building or other structure. Many HVAC systems include a controller that activates and deactivates one or more HVAC units or components of the HVAC system to affect and control one or more environmental conditions within the building. These environmental conditions can include, but are not limited to, temperature, humidity, and/or ventilation. In many cases, such an HVAC controller may include, or have access to, one or more sensors, and may use parameters provided by the one or more sensors to control the one or more HVAC components to achieve desired programmed or set environmental conditions.
An HVAC controller may be equipped with a user interface that allows a user to monitor and adjust the environmental conditions at one or more locations within the building. With more modern designs, the interface typically includes a display panel, such as a liquid crystal display panel, inset within a housing that contains a microprocessor as well as other components of the HVAC controller. In some designs, the display panel may be a touch sensitive display panel structured to accept user input. In some designs, the user interface may permit the user to program the controller to activate on a certain schedule determined by the user. For example, the interface may include a routine that permits the user to change the temperature at one or more times during a particular day and/or group of days. Such a programmable schedule may help reduce energy consumption of the HVAC system by changing the setpoint to an energy saving set back temperature during certain times, such as when the building or space is expected to be unoccupied or when the occupants are expected to be sleeping.
An HVAC system may be equipped to effect temperature changes in a building in multiple modes and/or with multiple stages. For example, an HVAC system may include a heat pump to extract heat from outside air and/or a geothermal reservoir and deliver the heat to a residence as a primary source, and also include auxiliary electrical resistance heaters to provide heat to the residence when the heat pump and/or other source is unable to deliver sufficient heat. In such an example, the auxiliary heat may be significantly more expensive to operate than the primary heat. In another example, a heating unit such as a furnace may include multiple stages, such a low-fire stage and a high-fire state. The cost per unit of heat delivered may or may not be greater for the high-fire stage as compared with the low fire stage. For consistency in nomenclature, an “auxiliary” source of heating or cooling may be used in this disclosure to denote a device, stage, mode, etc. of an HVAC system that provides heating or cooling at a greater unit cost than another source in the system, which may be described as a “primary” source.
At least three scenarios or conditions may be envisioned when the use of auxiliary heat may be desired. A first situation is when the HVAC system is tasked with maintaining a steady state temperature, but the primary heat source is unable to keep up with heat loss from the space, and thus, auxiliary heat is called upon to supplement or replace the primary heat. A second case is when a setpoint is manually increased. The primary heat source may be capable of moving the temperature to the new setpoint, but the time required to do so may be considered unacceptably long, in which case an auxiliary heat may be called upon to hasten the temperature change. A third scenario may occur when the system is responding to a programmed setpoint change (e.g., from a “sleep” to “wake” period of a programmed schedule), and the primary heat source may be unable to move the temperature to the higher setpoint by the desired time.
Because auxiliary heat may be more expensive than primary or lower-stage heat, a homeowner/building superintendent/etc. may be willing to sacrifice some comfort in favor of economy if it is possible to do so by avoiding and/or delaying the use of auxiliary heat. There is a need for improved devices and control methods to manage this kind of trade-off between comfort and economy.